The present invention relates to a rotary fluid pressure cylinder device to be used for operating a power chuck for a machine tool.
Recently lathe spindles rotate at a remarkably high speed for a high speed cutting. The high speed cutting causes heat generation of a rotary fluid cylinder device in use for the operation of a power chuck. As such a cylinder device, there has been generally used one operated by means of hydraulic or pneumatic power. Particularly, in the case of using the hydraulic power, as hydraulic pressure is supplied to a rotating cylinder, the heat generation at a rotary fluid coupling part is great. As a principal cause thereof, the following matters may well be mentioned. Namely, the hydraulic pressure is supplied to the rotary fluid cylinder to move a chuck coupled therewith for holding a workpiece in a state wherein the rotation of the cylinder is stopped. The hydraulic pressure must be supplied thereto for the purpose of safety or for keeping the force of chuck also when it is rotating to perform a cutting operation. This pressure medium, that is, a hydraulic oil having a large viscosity, leaks into small clearances between the inner wall surface of a stationary casing and the surface of a rotary member in a rotary fluid coupling whereby the oil which is filled therebetween is subjected to a viscous friction due to a high circumferential speed of the rotary member surface thereby to generate heat. This heat generation is known to be increased in proportion to a square of the speed of the rotary member.
As one of countermeasures to the foregoing, there has been already proposed a method disclosed in Japanese Patent Laid-Open Publication No. 55-152991 (corresponding to German Patent Application No. 2919180.8). In this method, the relative speed between respective wall surfaces defining clearances is reduced. However, as small clearances are disposed double, the leakage loss increases. This leakage loss is also a cause of the heat generation next to that due to the viscous friction and this heat generation is based on transformation of the mechanical energy of the hydraulic oil to heat while the oil leaks into the clearances. The heat generation raises the temperature of the entirety of the device. As a countermeasures thereto, it may be considered reasonable to devise a betterment in the precision of parts of the cylinder to thereby additionally minimize the clearances. However, in consideration of the known fact that in a small clearance the viscous friction increases in inverse proportion to a square of the largeness of the clearance, it is not an effective measure to counter the heat generation.
Such heat generation as described above causes a thermal deformation of the rotary fluid cylinder, not only causing a reduction in the fluid efficiency, but also raising the temperature of the entirety of the device as aforementioned. Rising temperature has various detrimental results. To dispose of the heat generation, there are a number of available measures such as of equipping the cylinder with a cooling fan to thereby promote heat radiation, of inlaying heat pipes as heat exchanger members in heat generating portions to thereby transmit the heat outside, and of introducing atmospheric air near heat generating portions to thereby radiate a part of the heat, whereas none of such measures give a sufficient effect. Therefore, in those rotary fluid cylinder devices in which fluids are circulated, most of the devices employ particular cooling equipments.
From the view point of energy saving, such energy for cooling and the energy wasted by heat generation are all useless. In this respect, according to an experiment by the present inventor, when a hydraulic rotary cylinder with a through hole of a diameter of 120 mm is rotated under a hydraulic pressure of 25 kg/cm.sup.2, a mechanical energy of approximately 1.5 kW is changed into heat to be lost at 1,500 r.p.m. because of the viscous friction.